We have found that severe reactions to contrast material in both humans and experimental animals are characterized by certain changes produced in the complement and coagulation cascades. These changes are found largely in proximal components of these two cascades, and in previous studies we have indicated a major role for the Cl-esterase inhibitor. Recently, we have found that pretreating animals with adrenalcorticosteroids produces elevation of the Cl-esterase inhibitor and this elevation evidently confers protection against contrast challenge. We propose to explore the mechanism whereby steroids produce these changes. To do this, we will examine changes in Cl, in prekallikrein and kallikrein, and in high molecular weight kininogen before and after steroid administration. We will also determine the direct protective effects, if any, of priming rabbits with human Cl-esterase inhibitor prior to contrast challenge.